This invention relates generally to the production of multilayered components, and more particularly concerns process and apparatus to produce multilayered electrical components; additionally, the products produced by the process are part of the invention.
Conventional processes for producing commercial multilayer capacitors employ the following steps:
1. Casting a ceramic slip by use of a doctor blade to form a green, dried ceramic film of 0.001" to 0.002" thickness;
2. Printing a registered matrix of metal pigmented inks to form the electrodes of the finished capacitor on the ceramic film;
3. Stacking a number of the registered electrode matrices in a cavity and laminating the stack of printed ceramic sheets with pressure and heat to form a compacted structure;
4. Cutting the compacted structure as by use of a guillotine type cutter.
The number of parts generated is determined by the number of electrodes in the printing matrix;
5. Thermal processing consists of a drying and bake out cycle to eliminate the organic components from the green parts, followed by a firing cycle to 2,000.degree. F. to 2,300.degree. F. to form the final ceramic structure.
6. Metallizing the ends of each individual capacitor element is necessary to achieve the desired electronic configuration. This is accomplished by applying a small amount of a fritted silver paint to each end of the ceramic capacitor element. After both ends are dried, the parts are fired to form metallic surfaces by which the appropriate individual electrodes within the ceramic are interconnected, and also by which the finished part may be connected to an electronic circuit.
7. Testing for the various electrical parameters completes the manufacturing process.
The controls necessary to achieve a satisfactory yield of capacitors of a specified value are indicated by the mathematical relationships related in the design equation: EQU C=0.224(nkA/d)pf (1)
where,
C=capacitance of the device PA1 n=number of active layers PA1 k=dielectric constant of ceramic film PA1 A=active area of an electrode (fired) PA1 d=fired thickness of the dielectric film (in thousandth of an inch)
To achieve a given value for capacitance C one must accurately control values of these parameters, as follows:
(d) Dielectric thickness (typically 0.0013".+-.0.0001"), and
(k) Dielectric constant. Control of this parameter is not only related to "lots" (i.e. differently fired groups) but also requires a very carefully controlled firing profile for consistant results. "Lot" k values are statistically determined before releasing material to production. A number of ceramic formulations are used, each with its own unique configuration of electrical parameters. They usually are referred to as "bodies" i.e. k1200 body would be a ceramic whose k is 1200.
(A) The active area of the electrode. In this regard, the electrode configuration is usually a function of mechanical constraints since it sets the size of the capacitor. Controls relating to the electrode consist of using the lowest cost precious metal electrode alloy consistent with the processing temperature and body chemistry, and controlling the electrode thickness. In this regard, changes in thickness cause a second order effect on capacitance. Also, if the electrode material is too thin as applied, areas of the electrode may be non-conductive and the effective area A will be lowered.
(n) Number of active layers is important, in that once the size of the capacitor (length and width) has been set by space available, and the dielectric type and thickness are chosen as a function of the electrical circuit requirements, the number of layers (n) can be adjusted to achieve the design capacitance. Clearly, there are limits to the least and most capacitance available. The upper limit of "n" for a given part type is somewhere around 40 layers, since yield of good parts starts declining rapidly beyond that. Many parts with more layers are sold however, since high capacitance coupled with small size of a part is a premium condition and commands higher prices. It is difficult to maintain uniform, undistorted internal structures in these high layer parts because of the green ceramic density variations introduced in the manufacturing process. These result in shrinkage variations upon firing, which produce material distortions appearing as delaminations of the layered structure of the capacitor. This is the most serious mechanical defect which results from conventional production of multilayer capacitors, and one for which there is no non-destructive test available. If a production lot is sampled by making petrographic tests, and it is found that delaminations are occuring above a certain percentage (it varies as a function of end use), the whole lot must be scrapped.